The present invention relates to MEMS planar gyroscopes for sensing a rate of inertial rotation around at least one axis.
MEMS planar gyroscopes based on two counter oscillating masses are described, for example, in U.S. Pat. No. 7,243,542. They are often referred to as Tuning-Fork-Gyroscopes (TFG). A TFG has two main vibration modes: an Excitation mode in which the two masses are counter oscillating in the device plane and constitute an in-plane primary resonator, and a Coriolis mode in which the two masses constitute a secondary resonator which responds to Coriolis forces. The secondary resonator can be either in-plane or out-of-plane. Coriolis forces result from interaction of the measured inertial rotation rate and the gyroscope primary resonator periodic velocity. The secondary mode—also referred to as the Coriolis mode—can be perpendicular to the device (gyroscope) plane, or out of this plane. The Coriolis forces which are indicative of the inertial rotation rate can be measured by two methods well known to those in the art: open loop operation based on sensing the amplitude and phase of the secondary resonator using a position pickoff, and closed-loop (force-balance) operation based on deriving from the position pickoff control signals that are used to generate forces which act on the secondary resonator and oppose the Coriolis forces. The present invention can be implemented using either method.
The motion of the two masses that constitute the Coriolis resonator in response to inertial rate are ideally equal and opposite, while their responses to external linear vibrations are the same. If this condition is met then the difference between their motions in response to external vibrations is zero (common mode) while the Coriolis induced motions (differential mode) are added.
In prior art TFG, each of the two masses in combination with their supporting springs constitutes an individual mechanical Coriolis sub resonator with some mutual mechanical coupling. As a result the resonant frequency of each sub resonator is principally determined by its respective mass and springs and to some extent by the other resonator. If, due to mechanical manufacturing tolerances, the resonant frequencies of the two masses are not perfectly matched, they will respond differently to linear vibration and the difference between their responses will result in an erroneous reading under vibration conditions—see for example U.S. Pat. No. 7,043,985.
Another disadvantage of prior art TFGs is that the vibrating structure is supported by more than a single anchor region, or point; typically 2 or 4 regions. For example, the TFG described in U.S. Pat. Nos. 7,043,985 and 5,349,855 are symmetrical in both X and Y axes but the vibrating structure is supported by 2 widely separated anchor regions. In both patents, the Coriolis sub resonators are only lightly coupled. Similarly, the TFG described in U.S. Pat. Nos. 7,243,542 and 6,571,630 are supported by 4 anchor regions.
The disadvantage of supporting the vibrating structure by more than a single anchor is that stress is induced in the TFG device layer in response to temperature, as a result of differential thermal expansion between the substrate layer (e.g. glass) and the Silicon vibrating structure attached to it. This stress is proportional to the difference in expansion coefficient and to the separation between the anchor points. Ideally this distance is zero, i.e., all anchor points converge to a single point.
A further disadvantage of prior art TFG is that the springs that support the vibrating masses serve both primary and secondary resonators and cannot be optimized separately—see below.